Th17 cells provide critical antimicrobial defense at barrier surfaces, but dysregulated Th17 cell activation leads
to chronic tissue damage exemplified by autoimmune diseases including multiple sclerosis (MS). TCR and
costimulatory signaling strength are established players in T cell fate specification, and costimulation modulators
are being used for autoimmunity and cancer therapeutics. We showed in primary human T cells that CD28
signaling negatively tunes Th17 differentiation through pAkt in a dose-dependent manner. Mouse and human
CD28 cytoplasmic tails are highly conserved, but the proline-rich motif is followed by an additional proline in
humans that enhances CD28 signaling compared to mice (which have alanine). We generated a novel CD28
‘humanized’ CD28A210P mouse and show that thymic T cell development is unaffected but pAkt is increased and
Th17 differentiation and function are reduced, providing a novel tool to investigate CD28-mediated regulation of
Th17 function by enhancing rather than deleting CD28 signaling. We have identified one of the targets of CD28
regulation as the kinase PIP5K1a, generating nuclear PIP2. This led to the discovery that PIP5K1a promotes IL-
17 protein expression through a unique post-transcriptional mechanism involving the mRNA nuclear cap-binding
protein ARS2. Excitingly, inhibition of PIP5K1a blocked the IL-17 response to the autoantigen MBP in T cells
from people with Multiple Sclerosis. Our new data show that ARS2 deletion in effector Th17 cells prevents
disease in the mouse model of multiple sclerosis, EAE, and suggest additional roles for ARS2 in driving
pathogenic effector Th17 cells beyond IL-17 production. This proposal will explore these novel mechanisms of
post-transcriptional regulation of inflammatory Th17 expression to test the hypothesis that PIP5K1a-ARS2
mediated translation is a critical rheostat regulated by CD28 signaling that determines functional outcomes in
Th17 cells in the following aims. Aim 1: How is the PIP5K1a-ARS2 interaction regulated in differentiating Th17
cells? Aim 2: How does PIP5K1a-ARS2 axis regulate Th17 protein expression? Aim 3 Determine therapeutic
potential of targeting PIP5K1a and ARS2 in effector Th17 inflammatory responses in vivo. Together, these data
will provide much-needed insight into molecular mechanisms that drive inflammatory Th17 cell functions and
could be targeted therapeutically. This novel pathway of post-transcriptional regulation of inflammatory Th17 cell
protein expression was first identified in human Th17 cells, and has now been reverse-translated into validated
mouse models that allow detailed molecular dissection and functional testing in clinically-relevant disease
settings.